As the planet has gotten warmer, sea levels have been slowly rising at an average rate of 1.8 mm per year since 1961. The higher levels are caused by thermal expansion as well as from melting land-based ice. Most eyes have been on Greenland, the large arctic island covered with an immense ice sheet, as the critical source of melting ice. However, a new study has recently been published which suggests that Greenland is not as big a concern as the continent of Antarctica, and in particular, the West Antarctic ice sheet.

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The study was conducted by scientists at the University of Wisconsin-Madison and led by geoscience professor Anders Carlson. It was published on July 29 in Science magazine. They analyzed pre-historic trends in the Earth's climate to find clues on how the climate will react in the future.

About 125,000 years ago was the last interglacial period, when global temperatures were very high, causing a sea level rise of 13-20 feet. The researchers originally believed that Greenland was the overwhelming culprit for all the extra water simply because it has a vast ice sheet which stretches to lower latitudes than Antarctica. Also, they believed the Antarctic ice sheet was far more stable since it was much larger.

Carlson's research showed that it was actually the Antarctic ice sheet, and particularly, the West Antarctic ice sheet that accounted for half of all sea level rise 125,000 years ago. This suggests that melting Antarctic will also contribute heavily to sea level rise in the coming centuries.

The study was conducted by analyzing silt from an ocean floor core taken off the southern tip of Greenland. This area receives sediments carried off the island by melt-water streams. Using radiogenic isotopes, they identified the source of the sediments, tracking them back to one of three regions, each with its own geochemical signature. What they found was that all three regions were supplying sediment during the last interglacial period. This suggests that all the regions in Greenland still had ice cover.

"The ice definitely retreated to smaller than present extent and definitely raised sea level to higher than present" and continued to melt throughout the warm period, Carlson said. He also indicated that "the ice sheet seems to be more stable than some of the greater retreat values that people have presented."

The research suggests that the melting Antarctic was responsible for most of the sea level rise. "The implication of our results is that West Antarctica likely was much smaller than it is today. If West Antarctica collapsed, that means it's more unstable than we expected, which is quite scary."